The body produces at least 80 percent of its vitamin D upon exposure to sunlight.

But now vitamin D deficiency has become a sign of affluence. Children are spending more and more time indoors, and parents concerned about skin cancer are increasingly applying more and more powerful sunscreens to their kids before letting them go outside. As many as 70 percent of all British children may now suffer from vitamin D deficiency, and one in 1,000 has been diagnosed with rickets.